Dual Isotopic Composition Of Nitrate Research Articles

Tracing the contributions of different nitrate sources associated with submarine groundwater discharge in coastal seawaters off Jeju Island, Korea

We measured the concentrations of dissolved inorganic nutrients and the dual isotopic composition of nitrate (δ15N-NO3− and δ18O-NO3−) in coastal waters off Jeju, a volcanic island in Korea, to trace its main sources. Sampling of seawater and fresh groundwater was conducted in four different coastal areas of Jeju Island: Haengwon (HW), Pyoseon (PS), Ilgwa (IG), and Sagye (SG) in May 2020 and 2021. The significant negative correlations between NO3− and salinity in the four areas indicate that the main source of NO3− is fresh submarine groundwater discharge (FSGD), with the extrapolated fresh groundwater endmember values ranging from 170 to 300 μM (δ15N-NO3−: 4.1–10.8 and δ18O-NO3−: 1.7–6.4). The actual sources of SGD-driven NO3− in these coastal waters were determined using a bi-plot diagram (δ15N-NO3− vs. δ18O-NO3−) and a Bayesian stable isotope mixing model (MixSIAR). The results showed that, besides the background contribution from open-ocean waters, the main sources of NO3− in HW were fertilizer (69 ± 5%) and manure and sewage (24 ± 7%) and those in PS, IG, and SG were manure and sewage (PS: 53 ± 11%, IG: 57 ± 12%, SG: 63 ± 13%) and fertilizer (PS: 27 ± 8%, IG: 24 ± 5%, SG: 22 ± 5%). Our extrapolation approach for NO3− dual isotopes provides a better way to evaluate the main sources of NO3− in coastal waters off volcanic islands where SGD-driven NO3− is significant, but its actual source groundwater cannot be located.

Identifying the sources of nitrate contamination using a combined dual isotope, chemical and Bayesian model approach in a tropical agricultural river: Case study in the Mun River, Thailand

Environmental issues triggered by increasing nitrate in agricultural river has become global concern. Identifying nitrate sources and transformation are crucial for water sources protection and eliminating nitrate contamination in an agricultural watershed. In this study, chemical and dual isotopic compositions of nitrate were employed to trace the nitrate sources and transformation processes, and proportional contribution of NO3− source were estimated by SIAR based on Bayesian model. NH4+ concentrations in middle Mun and lower Mun in wet season were significantly higher than NO3−, suggesting enhanced runoff processes by flood promote agricultural fertilized NH4+ leaching into the river. Higher Cl− concentration and NO3−/Cl− indicated that manure and sewage was the dominate nitrate source in the Lam Takhong River and the upper Mun. The overall values of δ15N-NO3− and δ18O-NO3− ranged from −3.9‰ to +16.6‰ and from −5.2‰ to +40.0‰, respectively. The results of nitrate isotopes indicated that NO3− mainly originated from soil N nitrogen, chemical fertilizer, and manure and sewage wastes. Spatially, soil N and chemical fertilizer contributed the most nitrate in the mainstream of lower Mun, middle Mun, and the Lam Takhong River; whereas over 60% of nitrate was derived from manure and sewage in the upper Mun. The spatial variation of water discharge and rainfall, together with the nitrate concentration and isotopes inferred that the nitrate sources and transformations in rain-fed river in tropical zone were distinguished from other rivers. High water discharge driven by rainfall events accelerated the nitrate export and the contribution of atmospheric deposition in wet season, and enlarged the contribution of manure and sewage in dry season. This study provided an example for further researches and approaches to assess the effects of tropical climate and agriculture on nitrate accumulation in watershed.

Tracing nitrate source and transformation in a semiarid loess aquifer with the thick unsaturated zone

The unsaturated zone (UZ) plays an important link between groundwater and the land surface. The surface pollutants would be retained firstly in the UZ when the UZ is thick and the recharge rate is limited before they percolate into groundwater. This study is dedicated to revealing the groundwater nitrate source and transformation by combining evaluations of the thick UZ (40–75 m) with the saturated zone in a loess aquifer using dual isotopic composition of nitrate from precipitation, soil water, and groundwater. According to the percolation velocity of soil water (0.12–0.14 m/yr), the depth above and below 10 m is taken as the shallow and deep part of the UZ, respectively. Results show that recent human activities have already caused nitrate pollution in the shallow part, which originated from chemical fertilizers, manure, and soil nitrogen. However, the nitrate in the deep part mainly originates from soil nitrogen. Denitrification in the shallow and deep part of the UZ is not evident. The groundwaters are all tritium-free (i.e., recharged before the 1950s), and therefore the water quality has not been affected by recent human activities. The groundwater nitrate concentration ranges from 6.5 mg/L to 19.2 mg/L with an average of 12.6 mg/L and mainly originates from the nitrification of soil nitrogen. Denitrification process is also not evident in groundwater, suggesting that the hydrogeological condition (especially the buffering effect of the thick UZ) controls the migration of nitrate. However, nitrate produced by human activities on the land surface would enter the aquifer eventually, posing a potential risk on water quality in the future.

The Dual Isotopic Composition of Nitrate in the Gulf of Mexico and Florida Straits

AbstractThe Gulf of Mexico receives considerable riverine nutrient inputs that contribute to eutrophication of northern shelf waters. However, it is unclear whether a significant fraction of these riverine nutrients delivered to the Gulf of Mexico are subsequently exported from the Gulf, potentially influencing the chemistry of the North Atlantic. Here, we report the first dual isotopic composition (“δ15N” and “δ18O”) measurements of nitrate (NO3−) from three profiles collected in different years in the Gulf of Mexico, as well as a new profile from the Florida Straits. We compare these new measurements with prior measurements from the North and South Atlantic, as well as with the magnitude of riverine and other nitrogen fluxes to the Gulf of Mexico. We find that in the upper thermocline, NO3− δ15N measurements from the Gulf of Mexico are intermediate between prior measurements in the Sargasso Sea (lower in NO3− δ15N), and those from the South Atlantic (higher in NO3− δ15N), but are indistinguishable from Florida Straits and Sargasso Sea profiles on deeper isopycnal surfaces. There is little evidence for riverine NO3− influencing the isotopic composition of NO3− exported from the Gulf of Mexico, which, based on its δ15N, would be expected to raise the δ15N of shallow NO3−. Lesser nitrogen fluxes are too small to significantly influence the isotopic composition of water column NO3− in the Gulf of Mexico. These results are consistent with recent modeling work suggesting that the majority of riverine nutrients are retained in the nearshore region.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate

Nitrate (NO3−) has become recognized as the most important water-soluble ion in fine particulate (PM2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO3−concentration, δ18O-NO3− and δ15N-NO3− values in PM2.5 were analyzed. NO3− concentrations increased as PM2.5 levels increased during both polluted and non-polluted days over the entire year. δ18O-NO3− values during cold months (63.5–103‰) were higher than those during warm months (50.3–85.4‰), these results suggested that the nitrate formation pathways shifted from the NO2 + OH (POH) in warm months to the N2O5 + H2O (PN2O5) and NO3 + VOCs (PNO3) pathways in cold months. Especially during cold months, δ18O-NO3− values increased from 65.2–79.9‰ to 80.7–96.2‰ when PM2.5 increased from ∼25 to >100 μg/m3, but when PM2.5 > 100 μg/m3, there were relatively small variations in δ18O-NO3−. These results suggested that nitrate formation pathways changed from POH to PN2O5 and PNO3 pathways when PM2.5 < 100 μg/m3, but that PN2O5 and PNO3 dominated nitrate production when PM2.5 > 100 μg/m3. Higher δ15N-NO3− values in warm months (−11.8–13.8‰) than in cold months (−0.7–22.6‰) may be attributed to differences in NOx emission sources and nitrogen isotopic fractionation among NOx and NO3−. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM2.5 levels.

Tracing the Origin of Groundwater Nitrate in an Area Affected by Acid Rain Using Dual Isotopic Composition of Nitrate

Acid rain with a relatively high concentration of ammonium and nitrate can accelerate rock weathering. However, its impact on groundwater nitrate is uncertain. This study evaluated the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) from precipitation to groundwater in a rural mountainous area affected by acid rain. The average concentration for NH4+ is 1.25 mg/L and NO3- is 2.59 mg/L of acid rain. Groundwater NO3- concentrations ranged from &lt;0.05 to 11.8 mg/L (baseline), and NH4+ concentrations ranged from 0.06 to 0.28 mg/L. The results show that groundwater δ18O-NO3- values (-4.7‰ to +4.2‰) were lower than the values of rainfall δ18O-NO3- (+24.9‰ to +67.3‰), suggesting that rainfall NO3- contributes little to groundwater NO3-. Groundwater δ15N-NO3- values (+0.1‰ to +7.5‰) were higher than the values of δ15N-NO3- derived from the nitrification of rainfall NH4+ (less than -4.7‰ in the study area), suggesting that nitrification of rainfall NH4+ also contributes little to groundwater NO3-. This implies that rainfall NO3- and NH4+ have been utilized. The dual isotopic composition of nitrate shows that baseline groundwater NO3- is derived mainly from nitrification of soil nitrogen. The denitrification process is limited in the groundwater system. This study shows that the rainfall NO3- and NH4+ contribute little to groundwater NO3-, improving the understanding of the nitrogen cycle in areas with a high concentration of NH4+ and NO3- in rainfall.

Rayleigh based concept to track NOx emission sources in urban areas of China

Despite the implementation of some effective measures to control emissions of nitrogen oxides (NOx = NO + NO2) in recent years, the ambient NOx concentration in urban cities of China remains high. Therefore, a quantitative understanding of NOx emission sources is critical to developing effective mediation policies. In the present study, the dual isotopic compositions of nitrate (p-NO3−) in fine-particle aerosol (PM2.5) collected daily at a regional scale (Beijing-Tianjin-Shijiazhuang) were measured to better constrain the NOx emission sources. The specific focus was on a typical haze episode that occurred simultaneously in the three urban cities (October 22–29, 2017). It was found that the nitrogen isotopic values of nitrate in PM2.5 (hereafter as δ15N-NO3−) ranged widely from −3.1‰ to + 11.4‰, with a mean value of 3.5 ± 3.7‰. Furthermore, a negative relationship between the δ15N-NO3− values and the corresponded p-NO3− concentrations during the haze period was observed. This implied the preferential formation of 15N-enriched NO3− into a fine-particle aerosol. Taking a different approach to previous publications, the Rayleigh fractionation model was used to characterize the initial isotopic signatures of ambient NOx. After accounting for the δ15N difference between p-NO3− and the source NOx, the estimated initial δ15N-NOx ranged from −20‰ to 0‰, which indicated a cosniderable contribution of non-fossil fuel emissions. The individual contributions of potential sources were further quantified using the Bayesian mixing model, revealing that NOx from coal or natural gas combustion, vehicle exhausts, biomass burning, and the microbial activity contributed 17.9 ± 11.4%, 29.4 ± 19.6%, 29.1 ± 18.9% and 23.5 ± 12.7% to NO3− in PM2.5, respectively. These results highlighted that tightening controls of gaseous NOx emissions from non-fossil sources may represent an opportunity to mitigate PM2.5 pollution in urban cities of China.

Geochemical, multi-isotopic studies and geothermal potential evaluation of the complex Djibouti volcanic aquifer (republic of Djibouti)

The complex Djibouti volcanic aquifer system was studied to improve understanding of the recharge conditions of the Awrlofoul low-enthalpy geothermal system located in the middle of the aquifer. Forty-four thermal and non-thermal groundwater samples were analyzed to determine their major chemical compositions, trace element compositions, and multi-isotopic compositions (δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ13C(DIC), 14C, 87Sr/86Sr, δ11B, δ15N(NO3−), and δ18O(NO3−)). Statistical analysis (Hierarchical Cluster Analysis and Principal Component Analysis) of chemical composition identified three main water groups, two affected by salinization (C1 and C2) and one fresh water group useful for drinking (C3). The latter group includes thermal water from the Awrlofoul geothermal field. This separation into three different water groups is also clear on a Langelier-Ludwig plot and is confirmed by analysis of historical chemical data over the last 30 years. The main causes of salinization are contamination of the fresh groundwater either by recent seawater intrusions (C2) or mixing with Ca-Cl fossil saline water (C1). The C1 waters are also highly affected by Mg/Ca-Na clay exchange. As expected, the 11B/10B isotope ratio of the intruded salt water, both recent and fossil, was much higher than that of seawater (δ11B up to +55‰). Unexpectedly, groundwater of meteoric origin (i.e., unaffected by a seawater intrusion), also showed a δ11B higher than that of seawater (46.3‰ < δ11B < 51.3‰). That the unexpectedly high δ11B values are likely due to 10B sequestration resulting from interaction with clay and/or carbonate precipitation is demonstrated by activity diagrams and saturation indices. The C1 water group is also affected by nitrate contamination (56.8 ± 19.2 mg/l). That the nitrate contamination is likely due to manure contamination is indicated by comparing the dual isotopic composition of nitrate to the boron isotope ratios. The isotopic composition of sulfate highlighted the importance of SO2-disproportionation to the local sulfate minerals that interacted with the meteoric recharge, while the strontium isotope ratios showed the importance of the seawater-basalt interaction with the fossil saline water component. The results of the mixing analysis using chemical composition, δ13C(DIC), and 14C data by geochemical software (NetpathXL) confirmed the presence of ternary mixing with at least three sources (seawater, meteoric, and fossil) in the waters with the highest chloride concentrations. The estimation of groundwater age by 14C was complicated by overexploitation (as testified by the lumped parameters approach). However, the fossil saline water component was dated back to the Holocene Humid Period.To estimate the temperature of the Awrlofoul low-enthalpy geothermal system, a multi-method geothermometric approach was applied. Chemical (mainly SiO2) and isotope (sulfate-water oxygen fractionation) geothermometers were employed together with multiple mineral equilibria. These different geothermometric approaches estimated a temperature range of 102 °C–140 °C for the geothermal reservoir, with a mean temperature of about 110 °C.Finally, a conceptual model was proposed for the Awrlofoul low-enthalpy geothermal system on the basis of the geochemical and isotopic data of the thermal and non-thermal groundwaters combined with the geology and hydrogeology of the study area.

Quantification of nitrate sources in groundwater using hydrochemical and dual isotopic data combined with a Bayesian mixing model

To determine the contributions of various sources to nitrate loads in groundwater (n=103) from three farmed rural sites in South Korea, we assessed the dual isotopic composition of nitrate, coupled with hydrochemical parameters. The measured isotopic values of nitrate varied widely, with 10th and 90th percentiles of 2.8‰ and 14.2‰ for δ15N, and 0.7‰ and 8.9‰ for δ18O, respectively and suggested that nitrification of anthropogenic N compounds is a predominant process of nitrate pollution. To overcome the known uncertainties of quantitative source apportionment for groundwater nitrate, arising from isotope fractionation during N transformations and the significant overlap in isotopic values of different sources, the combined hydrochemical and isotopic datasets were interpreted with orthogonal regression of a principal component analysis (PCA) and a Bayesian mixing model. The PCA projected the observed δ15NNO3 values onto a mixing subspace in the multivariate variability of the dataset, and the regression fits of the sample data were presumed to be conservative mixtures. This procedure also allowed for an assessment of the sample uncertainty, as influenced by natural nitrate contributions and denitrification. The Bayesian mixing model was used to estimate the probability distributions of the proportional contributions of three anthropogenic sources: chemical fertilizers, composted manure, and manure slurries. Nitrate is largely derived from chemical fertilizers with fractional contributions of 0.35–0.71, and organic fertilizers including composted manure with mixing fractions of 0.39–0.49. The relative contribution of nitrate from composted manure compared to chemical fertilizers increased with increasing nitrate concentrations, suggesting that composted manure significantly increases nitrate pollution and therefore its use should be carefully controlled to manage rural groundwater quality. This study also suggests that PCA and Bayesian isotope mixing models are effective for quantitative assessment of the sources of pollutants, such as nitrate.

Natural and anthropogenic sources and processes affecting water chemistry in two South Korean streams

Acid mine drainage (AMD) in a watershed provides potential sources of pollutants for surface and subsurface waters that can deteriorate water quality. Between March and early August 2011, water samples were collected from two streams in South Korea, one dominantly draining a watershed with carbonate bedrock affected by coal mines and another draining a watershed with silicate bedrock and a relatively undisturbed catchment area. The objective of the study was to identify the sources and processes controlling water chemistry, which was dependent on bedrock and land use. In the Odae stream (OS), the stream in the silicate-dominated catchment, Ca, Na, and HCO3 were the dominant ions and total dissolved solids (TDS) was low (26.1–165mg/L). In the Jijang stream (JS), in the carbonate-dominated watershed, TDS (224–434mg/L) and ion concentrations were typically higher, and Ca and SO4 were the dominant ions due to carbonate weathering and oxidation of pyrite exposed at coal mines. Dual isotopic compositions of sulfate (δ34SSO4 and δ18OSO4) verified that the SO4 in JS is derived mainly from sulfide mineral oxidation in coal mines. Cl in JS was highest upstream and decreased progressively downstream, which implies that pollutants from recreational facilities in the uppermost part of the catchment are the major source governing Cl concentrations within the discharge basin. Dual isotopic compositions of nitrate (δ15NNO3 and δ18ONO3) indicated that NO3 in JS is attributable to nitrification of soil organic matter but that NO3 in OS is derived mostly from manure. Additionally, the contributions of potential anthropogenic sources to the two streams were estimated in more detail by using a plot of δ34SSO4 and δ15NNO3. This study suggests that the dual isotope approach for sulfate and nitrate is an excellent additional tool for elucidating the sources and processes controlling the water chemistry of streams draining watersheds having different lithologies and land-use patterns.

Isotopic composition of water-soluble nitrate in bulk atmospheric deposition at Dongsha Island: sources and implications of external N supply to the northern South China Sea

Abstract. Increased reactive nitrogen (Nr, NO3− &amp;amp;plus; NH4&amp;amp;plus;+ &amp;amp;plus; dissolved organic nitrogen) emission from the Asian continent poses profound threats to ecosystem safety from terrestrial throughout the ocean proper. To quantify atmospheric Nr input, diagnose its sources, and evaluate influence on marine nitrogen cycle of the South China Sea (SCS), an oligotrophic marginal sea adjacent to the emission hot spot China, we conducted measurements of concentrations of nitrate and ammonium as well as other major ions and dual isotopes of nitrate (δ15NNO3 and δ18ONO3) in atmospheric deposition collected from Dongsha Island off southern China. The δ15NNO3 and δ18ONO3 for dry deposition averaged at −2.8‰ and +58.8‰, ranging from −7.5 to +3.7‰ and from ∼ +17 to +88‰, respectively. Wet deposition, although with limited samples, showed a similar flux-weighted mean in δ15NNO3 (−2.6‰) yet a significantly higher mean in δ18ONO3 (+78.8‰). The dual isotope ratios showed an anti-correlation and an inverse seasonality; the δ15NNO3 values were higher in summer compared to those in winter, while the δ18ONO3 values were higher in winter than those in summer. In winter, not only dual isotopic compositions of nitrate but also the ammonium and nitrate dry deposition fluxes were relatively uniform, demonstrating a persistent influence of fossil fuel combustion sourced from Asian continental outflows via the northeasterly monsoon winds. More variable isotopic values in summer likely suggest varying sources and dynamical formation processes of dry deposition nitrate. Biomass burning and lightning are suggested to be responsible for the observed higher δ15NNO3 values in summer. Atmospheric nitrate and ammonium deposition together was estimated to be ∼50 mmol N m−2 year−1, with the dominance of nitrate in dry deposition but ammonium slightly higher in wet deposition. If not including this additional fertilization of atmospheric inorganic nitrogen deposition to enhance the carbon sequestration, CO2 release out of the SCS would be double than that of the present amount, 460 ± 430 mmol C m−2 year−1. Our study demonstrates that atmospheric deposition may serve as an important external Nr supplier to the SCS; however it is difficult to separate the isotopic signal from N2 fixation (−2–0‰) due to their similarity in δ15NNO3. More studies related to isotopic composition of nitrogen speciation in atmospheric Nr deposition, their relative contributions and source identification, and the role of typhoons in the SCS are required.

An isotopic view on the connection between photolytic emissions of NOx from the Arctic snowpack and its oxidation by reactive halogens

We report on dual isotopic analyses (δ15N and Δ17O) of atmospheric nitrate at daily time‐resolution during the OASIS intensive field campaign at Barrow, Alaska, in March–April 2009. Such measurements allow for the examination of the coupling between snowpack emissions of nitrogen oxides (NOx = NO + NO2) and their involvement in reactive halogen‐mediated chemical reactions in the Arctic atmosphere. The measurements reveal that during the spring, lowδ15N values in atmospheric nitrate, indicative of snowpack emissions of NOx, are almost systematically associated with local oxidation of NOx by reactive halogens such as BrO, as indicated by 17O‐excess measurements (Δ17O). The high time‐resolution data from the intensive field campaign were complemented by weekly aerosol sampling between April 2009 and February 2010. The dual isotopic composition of nitrate (δ15N and Δ17O) obtained throughout this nearly full seasonal cycle is presented and compared to other seasonal‐scale measurements carried out in the Arctic and in non‐polar locations. In particular, the data allow for the investigation of the seasonal variations of reactive halogen chemistry and photochemical snowpack NOx emissions in the Arctic. In addition to the well characterized peak of snowpack NOx emissions during springtime in the Arctic (April to May), the data reveal that photochemical NOx emissions from the snowpack may also occur in other seasons as long as snow is present and there is sufficient UV radiation reaching the Earth's surface.

Spatial and temporal variations in groundwater nitrate at an intensive dairy farm in south-east Ireland: Insights from stable isotope data

Achievement of at least “good ecological status” in all waterbodies under the EU Water Framework Directive by 2015 will in some cases be a challenge. The twin challenge is to manage expectations of policy makers for such waterbodies as to a realistic length of time required for improvement in water quality. Hence, understanding the source, transformation processes and residence time of nitrate in a hydrological system is an essential part of meeting such challenges. On a dairy farm with 24 shallow groundwater wells, the dual isotopic composition of nitrate (δ 15N and δ 18O) was used to clarify nitrate sources, to assess spatial and temporal variability in nitrate concentrations and to determine if and where denitrification was occurring. Vertical travel time was estimated to correlate nitrate concentrations with management practices. Organically derived nitrogen was the predominant source contributing to groundwater nitrate concentrations. Denitrification was identified as prevalent within specific regions of the study site. The distinct low temporal variability in the isotopic data suggests constancy among nitrate sources and processes over time across the study site. Vertical travel times of up to 3 years were estimated on site indicating the influence of recent management practices on nitrate concentrations. Very slow horizontal migration of groundwater (decades) indicates a legacy of older management practices. Stable isotope techniques, together with an understanding of time lag, provide an extra mechanism to test the efficacy of monitoring and mitigation programmes.

Tracing sources of soil nitrate using the dual isotopic composition of nitrate in 2 M KCl-extracts

Soils comprise a critical interface between the atmosphere, lithosphere, hydrosphere and biosphere, and play a major role in the cycling of nitrogen (N), an element crucial to plant growth. Isotope techniques constitute a powerful tool to study the origin and fate of N compounds (e.g. NO 3 −) within the environment including soils. The objective of our study was to test the usefulness of the isotope composition of soil NO 3 − extracted with 2 M KCl (soil NO 3) as a tool to investigate the origin and fate of NO 3 − in the environment. Specifically issues related to repeat extractions, crop type, length of fertilization, and soil depth were addressed. Soils from four contrasting agricultural management regimes were sampled. Within the relatively confined study area (4 ha), the isotopic compositions of soil NO 3 differed markedly due to management treatments (up to 6 and 17‰ for δ 15N and δ 18O, respectively), but were repeatable among replicate plots (±1‰). Differences in both δ 15N and δ 18O values were observed between legume and non-legume treatments, as well as fertilized versus non-fertilized treatments, which were larger than the variability observed between replicate plots. Differences in the isotopic composition of extractable soil nitrate were not limited to the surface layer, but also occurred within deeper soil layers. This study indicates that the analysis of the natural abundance stable isotope composition of soil NO 3 may provide a promising additional tool for tracing the origin and fate of NO 3 − in the soil zone.

Nitrogen sources and cycling in the San Francisco Bay Estuary: A nitrate dual isotopic composition approach

We used the dual isotopic composition of nitrate (δ15N and δ18O) within the estuarine system of San Francisco (SF) Bay, California, to explore the utility of this approach for tracing sources and cycling of nitrate (NO2−). Surface water samples from 49 sites within the estuary were sampled during July–August 2004. Spatial variability in the isotopic composition suggests that there are multiple sources of nitrate to the bay ecosystem including seawater, several rivers and creeks, and sewage effluent. The spatial distribution of nitrate from these sources is heavily modulated by the hydrodynamics of the estuary. Mixing along the estuarine salinity gradient is the main control on the spatial variations in isotopic composition of nitrate within the northern arm of SF Bay. However, the nitrate isotopic composition in the southern arm of SF Bay exhibited a combination of source mixing and phytoplankton drawdown due mostly to the long residence time during the summer study period. Very low δ18ONO3 values (as low as −5.0%) at the Sacramento–San Joaquin River delta region give rise to a wide range of δ18ONO3 values in the SF Bay system. The range in δ18ONO3 values is more than twice that of δ15NNO3, suggesting that δ18ONO3 is an even more sensitive tool for tracing nitrate sources and cycling than δ15NNO3.